.gitlab-ci: Retrieve return value before any other command

This was previously introduced with commit '.gitlab.ci: Enable debug for
libsteat and for the DRM backend' in order to figure out another CI
issue we were seeing.

Unfortunatelly, not keeping the return value after the tests ran it
would silently make the entire CI succeed when it should actually fail.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>